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Disputed 'building block' of physics is constant

2 April 2004

By Maggie McKee

One of the fundamental constants that underpins physics has not changed as the Universe has evolved, according to a new analysis. The result is in sharp contrast to similar recent research suggesting the constant was growing.

“We really disagree,” says Patrick Petitjean, an astronomer at the Astrophysical Institute of Paris and a member of the team that did the new research.

The question is a vital one, he says&colon; “Physics is built on a few constants that do not change with time or space. You would have to introduce other dimensions of the Universe to explain any changes.”

Petitjean and his colleagues analysed data from the Very Large Telescope (VLT) in Chile to constrain the value of the fine-structure constant, which governs how electrically charged elementary particles and light interact. Their work shows it has changed no more than 0.6 parts per million – the resolution limit of the data – in the last 10 billion years.

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Research carried out at the Keck Observatory in Hawaii since the late 1990s indicated that the fine-structure constant has grown by as much as 5 parts per million over the last 12 billion years.

Speed of light

The fine-structure constant is an amalgam of other constants, including the charge of the electron and the speed of light – one or both of which would have to vary if the fine-structure constant was changing.

Petitjean’s team collected 34 nights of data from one of the VLT’s 8.2-metre dishes. They studied about 25 clouds of gas and dust that absorb light from distant, bright quasars. Subtle changes in the relative position of absorption lines from iron and magnesium in the clouds’ spectra would indicate changes in the fine-structure constant – but none were seen.

“It’s a difficult measurement, but the data we’ve got here are amazing,” Petitjean told New Scientist.

Michael Murphy, an astronomer at the University of Cambridge, UK, and a member of the team that found a varying value with Keck, agrees. “The VLT data is of higher quality,” he says. The Keck result was based on more clouds – 143 – but individual observations were done more quickly, and the quasars were on average fainter, making the spectral lines in the newer VLT measurements nearly twice as clear.

Different tack

Murphy says the analysis by Petitjean’s team is “comparatively rough but reasonable”. The Keck team are now using a slightly different approach to study the same VLT data and hope to report their independent calculation in about a year.

John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey, US, says Petitjean and his colleagues have done “an excellent piece of careful research”. He says their study uses “a better chosen sample, better quality data, and a cleaner way of analysing the spectra” than the previous Keck result.

Both Petitjean and Murphy say their groups are now attempting to resolve the discrepancy with new observational techniques and on different telescopes – but both acknowledge the measurement is a difficult one.

Bahcall agrees. “I do not expect the argument to be settled in the next decade,” he told New Scientist.

His own study of 165 quasars from the ongoing Sloan Digital Sky Survey suggests the constant has not changed in the last three billion years. Nevertheless, he says, “I suspect that [it] does vary with time … but the expected level of variation is well beyond current measurement techniques.”